The Identification of Intracellular Structures. 11 



o 



It is evident that the silver-nitrate reduction methods, not only 

 for the Golgi apparatus, but also for neuroglia and dendrites, are 

 extremely capricious, and care must be taken in interpreting tlie 

 images by these methods. The future training of zoologists will 

 not be complete unless they are taught one of the Golgi apparatus 

 methods of Golgi or Cajal, for the application of these methods to 

 germ cells has lately been giving very interesting results. The 

 Spanish observer Eio-Hortega, in a recent paper on the nuclei of 

 cells of the ovary and involuntary muscle of a mammal, describes 

 an enigmatic body which cannot be demonstrated by other methods. 

 This new nuclear body Eio-Hortega describes as a " nuclein fila- 

 ment"; it consists of a long filament or rod twisted up inside the 

 nucleus, and it is evidently separate from the chromatin of the 

 nucleus. Eio-Hortega does not describe the behaviour of this 

 body during mitosis. Achiicarro, another of Eamon y Cajal's 

 pupils, describes a new silver method for demonstrating neuroglia 

 ribres ; this new method gives remarkably reliable results, and has 

 been used in my laboratory with success. I believe that this method, 

 primarily invented and used by a neurohistologist, would gi\ e 

 useful results were it applied to " zoological material." 



I consider that after the various granules in the cell have been 

 exhaustively examined by silver and chrome-osmium techniques, 

 no further progress will be made in descriptive cytology unless 

 some new methods are introduced which will enable us to face the 

 problems before us from a different point. This especially applies 

 to the cytological basis of the questions of heredity. Should no 

 revolutionar}' cytological methods appear it would seem that de- 

 scriptive cytology will in the next few decades pass to the position 

 of present-day " Comparative Anatomy." 



Fixatives. 



For some years I have been working with a variety of fixatives 

 and methods, and give here some part of my new results. Fixation 

 falls under three broad headings : — 



1. Micro-anatomical, in which correct preservation of cell 

 aggregates, without shrinkage or expanding, is the desideratum. Such 

 is the aim of most embryologists. 



2. Oiitological from the chromosome or nucleus point of view. 



3. Cytological from the point of view of fixing the cell in a state 

 which most resembles its condition when alive ; also so as to 

 identify the cell elements, especially in the cytoplasm. 



Only the last section (No. 3) is treated here, as the other two 

 rare not a matter of difficulty. In most cases 'the results attairied 

 by workers belonging to sections 1 and 2 can truly be said to give 

 a caricature of the cell intra vitam. I give below a general classi- 

 fication of fixatives based mainly on the sections explained above. 



